1. Field of the Invention
The present invention pertains to an apparatus and method for driving a high intensity discharge (HID) lamp. Specifically, the present invention is directed to generating a high frequency resonant ignition voltage to ignite (start) the HID lamp, and to maintain a stable circuit operation with minimal interference from a high frequency resonant ignition circuit to a peak current sense signal that is used for output power and current control during a normal state of operation. A variation of the magnitude of the resonant ignition voltage with respect to a parasitic capacitance related to a length of the lamp leads is minimized by the inclusion of a damping resistor connected in series with a resonant ignition capacitor.
2. Discussion of Background and Relevant Information
Electronic high intensity discharge lamps generally employ one of two techniques for igniting (starting) the lamp. In a first technique, the lamp is ignited using a pulsed ignition method. In a second technique, the lamp is ignited using a resonant ignition method. The peak magnitude of the ignition voltage associated with the resonant ignition method is lower than the peak magnitude of the ignition voltage associated with the pulse ignition method. Accordingly, from the standpoint of safety, the resonant ignition method is generally preferred over the pulsed ignition method.
Further, two distinctively different methods exist to continue operating the lamp after it has been ignited. In a first method, the lamp is operated with a high frequency signal that is typically in the kilo Hertz (kHz) range. In a second method, the lamp is operated with a low frequency signal that is typically measured in the hundreds of Hertz range. Due to acoustic resonance problems associated with high frequency operations, it is generally preferred to employ the low frequency operation method to maintain the operation (e.g., illumination) of the lamp.
In order to generate a high frequency voltage having sufficient energy to ignite the lamp or to run the lamp (after ignition) with a high frequency signal, three fundamental approaches are generally taken, as shown in FIGS. 3(a) to 3(c).
FIG. 3(a) illustrates a discharge lamp driving circuit having a chopper and a high frequency inverter. Depending upon different control schemes applied to switches Q1 to Q4, this configuration can serve many design purposes.
It is known that HID lamps exhibit an acoustic resonance when operated at a high frequency. U.S. Pat. No. 4,912,374 discloses a method to interrupt the high frequency current with a smoothed DC current. Inductor L1 and capacitor C1 form a buck resonant network. Transformer T and capacitor C2 form an inverting resonant circuit. When transistor pair Q1 and Q4 and transistor pair Q2 and Q3 are alternately switched at a high frequency, two high frequency AC currents flow through the lamp. The first high frequency AC current is produced by the buck resonant network. The second high frequency AC current is produced by the inverting resonant network. As a result, a loop current is formed between the capacitor C1, the transformer T, and the lamp. When transistor Q4 is switched at a high frequency, transistor Q1 is ON, and transistors Q2 and Q3 are completely OFF (due to the chopper, or buck, configuration.), so that a DC current flows from left to right through the lamp. When transistor Q3 is switched at a high frequency, transistor Q2 is ON, and transistors Q1 and Q4 are completely OFF, so that a DC current flows from right to left through the lamp.
To control the DC current, some sort of buck current sensing is required. Such a system is not disclosed in detail in U.S. Pat. No. 4,912,374. The simplest method to sense the buck current is to add a sense resistor in series with input bus voltage V1. However, unless special care is taken to separate the inverting resonant network current from the buck network current, a coupling may occur between the inverting resonant network and the buck resonant network. U.S. Pat. No 4,912,374 does not disclose the separation of the inverting high frequency operation and the buck DC or low frequency operation, but the inverting high frequency operation is utilized just for starting (igniting) the lamp and the DC (or low frequency) operation is utilized for the normal (continuous) operation of the lamp after it has been started.
FIG. 3(b) illustrates a modification of U.S. Pat. No. 4,912,374, in which MOSFET Q5 and diode D5 are added. The inclusion of these components results in the lamp current comprising a clean square wave, while the sensed buck current comprises a clean triangular wave. It is noted that MOSFET Q5 can be switched OFF any time after the lamp is ignited (started), or whenever the high frequency current is not needed for the lamp operation. When MOSFET Q5 is switched OFF, the buck network, formed by inductor L1 and capacitor C1, and the ignition network, formed by transformer T and ignition capacitor C2, are completely decoupled. That is, ignition capacitor C2 is electrically disconnected from the circuit. There is no charge (or discharge) current flowing through the ignition capacitor C2 or the current sensing resistor Rs, due to the switching of transistors Q1 and Q2. Further, diode D5 prevents any voltage overshoot during the switching of MOSFET Q5.
The disadvantage of this modification is that a high voltage MOSFET Q5 and a high voltage diode D5 is required, along with any associated driving circuitry required to drive MOSFET Q5. This increases the circuit complexity and increases the manufacturing costs. It is noted that if the composite waveform of the high frequency current and the DC current is required to prevent an acoustic resonance, MOSFET Q5 has to be turned ON during the high frequency period and turned OFF during the low frequency period.
A dual stage output filter of U.S. Pat. No. 6,020,691 is illustrated in FIG. 3(c), in which a chopper (or buck) power regulator with a high frequency resonant ignition, a discontinuous first resonant stage inductor current, and a continuous second resonant stage inductor current are related to each other.
In U.S. Pat. No. 6,020,691, a first stage resonant frequency fr1, formed by inductor L1 and capacitor C1, is lower than a second stage resonant frequency fr2, formed by inductor L2 and capacitor C2. In addition, a distance between the first stage resonant frequency fr1 and the second stage resonant frequency fr2 is somewhat confined not to be less than a selected minimum value, in order to avoid an excessive resonant current circulating in the circuit. The ignition voltage is generated by sweeping the frequency over the second stage resonant frequency, fr2. For example, if the second stage resonant frequency fr2 is selected to be, for example, approximately 40 kHz and a minimum sweeping frequency is selected to be, for example, approximately 30 kHz, the first stage resonant frequency fr1 may be selected to be, for example, approximately 22 kHz. This kind of circuit arrangement suffers from frequency inaccuracies and component tolerance problems, because the circulating current of the first stage resonant network is highly related to the frequency fr1 and the minimum sweeping frequency. A further disadvantage of this circuit arrangement is that the magnitude of the ignition pulse, which is mainly generated by the second stage network, is a function of both resonant frequencies, since two stages are cascaded together. The input voltage signal, with its frequency near the second stage resonant frequency, is damped by the first stage network and amplified by the second stage network. Thus, the Q factor of the second stage network has to be significantly high so that enough ignition voltage can be generated.
The present invention overcomes the inability of the prior art to electrically separate (isolate) the first resonant network design and the second resonant network design. According to the present invention, the ignition capacitor is isolated from the circuit to prevent a charge current (and/or discharge current) from interfering with a load current sense circuit.
According to a feature of the invention, a relatively xe2x80x9ccleanxe2x80x9d signal is provided to a current sensing circuit of a buck regulator, even when a relatively high spike current is fed to the ignition capacitor.
According to an advantage of the present invention, a damping device, such as, for example, a damping resistor, is provided, such that a variation of a peak ignition voltage that is generated is limited to a minimal parasitic capacitance, such as, for example, a few hundred pico-farads, at the output.
According to another object of the invention, leakage current through the path of a bypass diode is significantly less than the current flowing through the sensing resistor, so that the current sensing is not affected by the diode leakage current.
According to an object of the present invention, a discharge lamp driving circuit, comprises a tank circuit, and a DC-AC inverter. The tank circuit has two resonant networks, and a lamp driving connection. The lamp driving connection is electrically connected to a lamp. The DC-AC inverter is electrically connected to a voltage input and to the tank circuit. A first resonant network of the tank circuit delivers an alternating rectangular current during a normal operation mode, while a second resonant network of the tank circuit delivers an alternating resonant ignition voltage during a starting operation mode. The tank circuit is configured so that the second resonant network includes at least one damping resistor in series with at least one resonant capacitor.
According to a feature of the invention, the DC-AC inverter may be either a full bridge inverter or a half bridge inverter.
According to an advantage of the invention, the first resonant network comprises a capacitor and an inductor that are electrically connected in series, and the first resonant network is connected to an output of the bridge circuit.
According to another advantage of the invention, the second resonant network comprises an inductor, a capacitor and a damping device. The inductor, capacitor and damping device are electrically connected in series, with the second resonant network being connected to an output of the bridge inverter and the bypass device.
A further advantage of the invention resides in the lamp driving connection being electrically connected in series with an inductor of the second resonant network and an inductor of the first resonant network, with the serially connected lamp driving connection and inductor of the second resonant network being further connected in parallel with a capacitor of the first resonant network.
Another object of the present invention resides in a discharge lamp driving circuit that comprises a tank circuit and a DC-AC inverter. The tank circuit has a first resonant network, a second resonant network, and a lamp driving connection. The lamp driving connection is electrically connected to a lamp. The DC-AC inverter, which is electrically connected to a voltage input and to the tank circuit, includes a sensing device, a bypass device associated with the second resonant network and a bridge circuit. The sensing device operates to sense an amount of current of the first resonant network in the tank circuit, while the bypass device operates to decouple the current flow of the second resonant network from the current flow of the first resonant network.
According to an advantage of the invention, the sensing device, which may be, for example, a sensing resistor, is connected in parallel with the bypass device.
According to a feature of the invention, the DC-AC inverter includes a bridge inverter, such as, for example, a full bridge inverter or a half bridge inverter.
A still further feature of the invention resides in the first resonant network comprising a capacitor and an inductor that are electrically connected in series, the first resonant network being connected to an output of the bridge inverter.
Another feature of the invention pertains to the second resonant network comprising an inductor, a capacitor and a damping device. The inductor, capacitor, and damping device are electrically connected in series, the second resonant network being connected to an output of the bridge inverter and the bypass device.
It is noted that the lamp driving connection may be electrically connected in series with an inductor of the second resonant network and an inductor of the first resonant network. The serially connected lamp driving connection and inductor of the second resonant network being further connected in parallel with a capacitor of the first resonant network.
Further, the sensing device may be connected between a first input connection of the voltage input and a first input of a bridge inverter, a second input of the bridge inverter being connected to a second input connection of the voltage input.
According to another object of the invention, a discharge lamp driving circuit comprises a tank circuit and a DC-AC inverter. The tank circuit includes a first resonant network, a second resonant network, and a lamp driving connection, with the lamp driving connection being electrically connected to a lamp. The DCAC inverter is connected to a voltage input and the tank circuit. The first resonant network delivers an alternating rectangular current (which may having an operating frequency of less than approximately 1 kHz) to the lamp during a normal operation mode, while the second resonant network delivers an alternating resonant ignition voltage (which may have an operating frequency greater than approximately 20 kHz) to the lamp during a starting operation mode. Additionally, the second resonant network includes at least one damping resistor in series with at least one resonant capacitor. The DC-AC inverter includes a sensing device, a bypass device, and a bridge inverter. The sensing device senses a current flow of the first resonant network while the bypass device de-couples a current flow of the first resonant network and a current flow of the second resonant network.
According to a feature of the invention, the sensing device is connected in parallel with the bypass device, which may comprise, for example, two series connected diodes. The sensing device may be connected between a first input connection of the DC voltage input and a first input of a bridge inverter, while a second input of the bridge inverter is connected to a second input connection of the DC voltage input.
A junction of the two series connected diodes may be connected to the second resonant network. In addition, a leakage preventing device may be connected in series with at least one of the two series connected diodes. The leakage preventing device exhibits a resistance that is greater than a resistance of the sensing device. Preferably, the resistance of the leakage preventing device is equal to at least twenty times said resistance of the sensing device.
According to another feature of the invention, the DC-AC inverter may be a full bridge inverter or a half bridge inverter.
According to an advantage of the invention, the first resonant network may comprise a capacitor and an inductor that are electrically connected in series, with the first resonant network being connected to an output of the bridge inverter. Further, the second resonant network may comprise an inductor, a capacitor and a damping device, in which the inductor, the capacitor and the damping device are electrically connected in series, the second resonant network being connected to an output of the bridge inverter and the bypass device.
The lamp driving connection may be electrically connected in series with an inductor of the second resonant network and an inductor of the first resonant network, with the serially connected lamp driving connection and inductor of the second resonant network being further connected in parallel with a capacitor of the first resonant network.
According to a still further object of the invention, a method is disclosed for driving a discharge lamp. According to the method, a lamp is electrically connected to a lamp driving output of a tank circuit having a first resonant network and a second resonant network, with a voltage being input to the tank circuit from a DC-AC inverter. The DC-AC inverter is electrically connected to a voltage input. The DC-AC inverter includes a sensor, a bypass device associated with the second resonant network, and a bridge circuit. An amount of current flowing in the first resonant network of the tank circuit is monitored with the sensor, while the current flowing in the second resonant network is decoupled from a current flowing in the first resonant network with the bypass device.
According to a feature of the invention, the sensor may be connected in parallel with the bypass device. The bypass device may comprise two series connected diodes. In such a configuration, a leakage prevention device may be electrically connected in series with at least one of the two series connected diodes. The leakage prevention device preferably exhibits a resistance that is at least twenty times greater than a resistance of the sensing device.
An additional feature of the invention is the inclusion of a bridge inverter, the first resonant network including a capacitor and an inductor that are electrically connected in series, the first resonant network being connected to the output of the bridge inverter.
A still further feature of the invention is that the second resonant network includes an inductor, a capacitor and a damping device. The inductor, the capacitor, and the damping device are electrically connected to an output of the bridge inverter and the bypass device.
A still further object of the invention pertains to a method for driving a discharge lamp that is electrically connected to a lamp driving output of a tank circuit, in which the tank circuit has a first resonant network and a second resonant network, the voltage being input to the tank circuit from the DC-AC inverter, the second resonant network having at least one damping resistor in series with at least one resonant capacitor, and in which a DC-AC inverter. having a sensor, a bypass device, and a bridge circuit is electrically connected to the DC voltage input and to the tank circuit. The method comprises operating the tank circuit such that the second resonant network delivers an alternating current ignition voltage to the lamp during a startup operation mode. After a lapse of a predetermined time period, the tank circuit is operated such that the first resonant network delivers an alternating rectangular current to the lamp during a normal operation mode. A current flow in the first resonant network is sensed with the sensor, while a current flow in the second resonant network is decoupled from the current flow in the first resonant network with the bypass device of the DC-AC inverter.